This section provides background information related to the present disclosure which is not necessarily prior art.
There is a presently a strong interest in improving the economics of a capacitive deionization system which is used for water desalination. Water desalination is expected to continue growing in importance over the coming years as shortages of fresh water continue to be experienced in various regions around the world, and especially in those regions bordering bodies of salt water.
Capacitive deionization is one method of desalination which removes salt from salt water by using electric fields. However, due to finite charge capacity, the capacitors used in such systems, which act as electrodes, have to be charged and discharged in a continuous cycle of desalination and regeneration. This is done typically by transferring the charge between two capacitors, and only adding additional energy to the system as energy is resistively dissipated. In a typical capacitive deionization system, the desalination (removal of ions from the water) takes place when the capacitor is charging, and the regeneration (removal of salt from the capacitor) takes place when the electrode is discharging. While there are capacitive desalination systems in which the charge states are reversed, the charging step will be called the desalination step, and the discharging step will be called the regeneration step throughout this document.
The conventional means of performing this transfer is by using a buck boost converter. Such a circuit is shown in FIG. 1. In this circuit capacitor C1 acts as the first electrode (i.e., electrode 1) during the desalination operation to store charge as salt water moves between its plates and ions are removed from the water. This results in the buildup of a charge on electrode 1 (C1). When electrode 1 (C1) is fully charged, the circuit then begins to transfer energy from electrode 1 (C1) to an inductor (L1), and then from the inductor to a second electrode (i.e., electrode 2, labelled as C2). This transfer is done in two steps, and current passes through each electrode only half the time. Electrode 1 (C1) discharges when switch 1 (swc1) is closed (conducting) and switch 2 (swc2) is open. Current flows into the inductor (L1) and the inductor begins storing the energy from electrode 1 (C1) in its magnetic field. When the current through the inductor (L1) reaches a predetermined maximum, then switch 1 is opened and switch 2 is closed. At this point the inductor (L1) begins transferring its magnetic field charge energy through switch 2 to electrode 2 (C2). When the current flow in the inductor (L1) reaches a predetermined minimum, switch 2 is opened and switch 1 is closed, and the above cycle repeats itself. It typically takes numerous cycles to transfer all of the energy from electrode 1 (C1) to electrode 2 (C2). As such, each one of the electrodes 1 and 2 is only charging or discharging one half the time during any given charging or discharging cycle. This means that it takes twice as long to fully charge an electrode than it would if the charging process was continuous at the same average current. It also means that it takes twice as long to fully discharge one of the electrodes than it would if the discharging process was continuous. One method to speed up the transfer of charge from electrode 1 to the inductor, and then from the inductor to electrode 2, is to allow a greater current flow during the charge transfer process. Thus, using a higher current flow will improve the throughput of the system, but increasing the magnitude of current flow will also increase the power dissipation losses due to the internal resistances of the electrodes and the cables. Accordingly, simply increasing the current flow to reduce the time needed to complete the regeneration step is not a preferred option. It should also be noted that, under the current configuration of the circuit, half the capacitor material will be desalinating water at any given time, whereas the rest is regenerating, which means that half of the material is not contributing to the desalination operation under this configuration.
From the above it will be appreciated that limitations and tradeoffs exist in speeding up the transfer of charge from one capacitor to another. Since the desalination step cannot be initiated until the regeneration step is completed, any feature or improvement that reduces the time needed to perform the regeneration step (i.e., remove the stored charge from the capacitor that performs the desalination step) to another electrode, would improve the throughput of the system and thus its economic performance as well.